1. Field of the Invention
The present invention relates generally to imaging, and in particular, to a method, apparatus, and article of manufacture for fabricating and using a metallic glass transition-edge-sensor (MGTES) device.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Actively cooled, direct radiation detectors, which exhibit improved image and energy resolution in the infra-red (IR) and x-ray wavelength regions, are a next-generation technology for advanced imaging, for both terrestrial and space applications. In this regard, NASA's Science Mission Directorate (SMD) is driven in part by the motivation for understanding the content and evolution of matter in the known universe. To support these goals, a broad spectrum of missions are planned to conduct measurements at submillimeter and far-infrared (FIR) wavelengths; e.g., the NASA proposed Background-Limited far-IR/Submillimeter Spectrograph (BLISS) and the SpicA FAR Infrared Instrument (SAFARI) instruments for Japan's Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission. SPICA requires actively cooled, direct radiation detectors, with very high sensitivity. Potential terrestrial applications for actively cooled, direct radiation detectors arrays are: 1) x-ray detectors for medical imaging; 2) chemical analysis for materials science (Scanning-electron-microscopy/Energy Dispersive Spectroscopy) at x-ray wavelengths; and 3) radiation detectors for nuclear forensics (e.g., Alpha-particle and gamma-ray detection).
Membrane-isolated superconducting transition-edge sensor (TES) arrays are a leading sensor technology for such space applications (see [1]-[3]). To meet application requirements, the detectors must be formatted into large-scale high pixel density arrays, which will be read using superconducting quantum interference detector (SQUID) Multiplexer (MUX) technology. Detector arrays that use the current baseline transition edge sensor technology, i.e., state-of-the art (SOA) TES arrays, do not exhibit the performance requirements of the BLISS instrument. The BLISS instrument requires detectors with the following performance metrics: 1) fast response time τ (τ<100 mS); 2) high pixel density arrays (at least 103 pixels); 3) TES films must exhibit stable TC<100 mK; 4) must perform over a broad wavelength range 35 mm-433 mm; 5) low detector noise equivalent power (NEP) of the order NEP<1×10−19 W/√Hz; and 6) low 1/f noise level at low frequencies, f, below 10 Hz.
TES-based microcalorimeter arrays are being developed for the mission listed above, as well as the NASA Beyond Einstein Program Constellation-X Observatory (Con-X, for measurements of x-ray spectra). SOA TES devices detect radiation by precisely measuring the temperature rise associated with the absorption of a photon, which raises the TES temperature. The temperature increase is detected by a superconducting thermistor, which is voltage-biased 10-20% into the superconducting transition curve at T=TC. The superconducting thermistors in SOA TES devices are typically based on: 1) elemental superconducting films; e.g., Titanium (Ti, TC=565 mK) or Iridium (Ir, TC=130 mK); and 2) proximity effect elemental metallic bilayer design; e.g., Mo/Au. FIGS. 1A and 1B illustrate an ordinary bilayer Mo/Au (Molybdenum/Gold) TES (FIG. 1A) and TES with stripes (FIG. 1B) (140 microns on a side, stripes spaced every 15 μm) of the prior art. In the single-layer elemental TES designs, the superconducting transition temperature TC cannot be controlled, and is well above the desired operating temperature (˜50 mK) for space applications. The bilayer TES device functions as a single superconducting element via the proximity effect, and the superconducting transition temperature, TC, is controlled by the thickness of the superconductive film element. However, this TES device architecture has inherent problems, which are manifest in fabrication difficulties, control of the superconducting transition temperature, TC, and an excess noise equivalent power spectrum.
In other words, while the techniques used to fabricate SOA TES single-layer elemental and TES bilayer designs are well developed, the TES device architecture exhibits a variety of problems: 1) device fabrication is difficult to control, resulting in variable superconducting transition temperature, Tc, from element to element; 2) broadened transition widths, ΔTC˜2-30 mK; and 3) SOA TES devices exhibit an excess noise equivalent power spectrum (NEP) (see [4]-[5]).
SOA TES devices based on the bi-layer design are designed for operation at ˜50 mK, and the individual TES films must exhibit similar Tc values for good performance. In SOA TES devices, the Tc values are difficult to control because these devices are fabricated using thin-film synthesis techniques based on geometry. Achieving the control necessary to fabricate large arrays is technically challenging, as the TC is a sensitive function of the properties of both layers, as well as the interface transparency. In Mo/Au proximity effect bilayer TES designs, the lack of control of the residual resistance ratio (RRR) for as-sputtered Au films impacts the Tc value, and post-annealing to improve the RRR contribute to un-controlled changes in Tc.
The e-beam deposition techniques used to fabricate bilayer TES devices are also manifest with challenges. Typically, Mo/Au films are e-beam deposited onto an LSN (low-stress silicon nitride) substrate maintained at 600° C.; the high substrate temperature is used to compensate for thermal expansion coefficient mismatch between the metal film and substrate. The remaining residual stress in these Mo-films is of the order 400 to 600 MPa (Megapascals). In these films, the residual stress shifted the superconducting transition temperature Tc by as much as 50 mK.
SOA single-layer elemental design TES detectors; e.g., based on Ti or Ir, typically exhibit an excess noise-equivalent-power (NEP), which is 2× larger (or greater) than that predicted by theory. SOA bilayer design TES detectors typically exhibit an NEP which is 4-10× greater than that predicted by theory. FIG. 2 illustrates excess noise in bilayer TES compared to the calculated Johnson noise of the TES and shunt resistor in the prior art. The source of this noise is unknown. Resolution of the excess noise is a global challenge for everyone in the field.
In the SOA designs, there are factors intrinsic to their design, which may limit the reduction of the excess noise spectrum to acceptable levels. For example, in Mo/Au proximity effect bilayer TES designs, the residual stress state in the bilayers has ramifications on the excess noise exhibited by these films. Local fluctuations in the film stress state, on spatial length scales of the order of the superconducting coherence length ξ can increase quasiparticle carrier scattering in the metallic layer, which can introduce additional noise. Defects present in the elemental crystalline films (e.g., grain boundaries and dislocations) contribute to an increase in quasi-particle charge carrier scattering at low temperatures, thereby increasing the noise levels observed. Interlayer quasiparticle carrier scattering in metallic bilayers will also introduce additional noise.
In addition to the technical challenges outlined above, another issue that limits the application of TES devices for terrestrial applications, is the low TES operating temperature, Top˜100 mK. To actively cool TES arrays to this temperature range, complex and costly cryogenic cooling methods (e.g., dilution refrigerators) are required. A TES technology that would enable operation at 0.3 K or higher, would be very desirable, as this temperature can be achieved using closed-cycle refrigerator systems using 3He or the newer adiabatic demagnetization refrigerator (ADR) system (e.g., a continuous adiabatic demagnetization refrigerator (ADR with 4-stages), a He3/He4 dilution refrigerator, or a cryogenic cooling system that uses liquid 4He).
The performance limiting difficulties associated with the SOA TES devices are:                Tc controlled only by superconducting proximity effect in metallic bilayer design and for these ultra-thin films the substrate surface roughness broadens the transition; therefore Tc value is difficult to control;        Energy resolution limited by low logarithmic sensitivity factor (α) values, manifest by large transition widths, ΔTc;        Large excess noise values at high frequencies [1-20 kHz] (up to 20 times theoretical expectations), correspondingly reduces the high-frequency sensitivity;        Sources of large excess noise are unknown;        Fabrication techniques introduce wide variability in TES performance values;        SOA TES design itself introduces unavoidable extrinsic sources of excess noise; and        Expensive cryogenic technology required to achieve cooling to ˜100 mK.        